The project is aimed at measuring the spread of material through the subway system by collecting air samples after the gas and particles are released, according to DHS' summary of the study.

"A better understanding of the spread of these materials within the subway and the greater urban environment will help in characterizing hazardous material releases originating in the subway, whether they are intentional or accidental," DHS officials said.

The study also will allow researchers to measure the amount of particulate materials that settle on the ground and other surfaces.This, in turn, allows DHS to assess contamination and possible hazards in the initial phase of emergency response, as well as cleanup actions.

The study began Monday and will last through Friday. A single 20-minute release period of particle "tracers" occurs at 11 a.m. each day. Release locations include Grand Central, Times Square and Penn Station subway platforms.

Particulate and gas sampling will occur in about 55 other subway stations over a four-hour window following the tracer releases.

The department is using safe, nontoxic gas and particle tracers that are colorless, odorless and biologically inert.

The study is part of a larger five-year program to develop and promote technologies and methods to help urban transit systems restore service rapidly in the event of a biological attack, DHS officials said.